The present invention relates to the use of very low friction material formed into patches or pieces and adhered to the skin or to a surface in contact with the skin (or immediately adjacent material such as a sock) to lower the magnitude of tangential traction of the surface in contact with the skin. The material reduces the likelihood of abrasion, trauma and ulceration in localized areas.
In the prior art, there have been efforts to reduce the co-efficient of friction of materials in load bearing contact with the skin, such as the surface of a lining of a shoe, which slides against a stocking. Also the regions where a limb prosthesis is in load bearing contact with a residual limb have been extensively considered for ways of reducing problems. The co-efficient of friction of smooth leather varies, depending on the moisture content, and when it gets wet can be quite high in friction. Moleskin patches have been sold and used for covering corns on the feet, as well as covering calluses, but this also has a relatively high co-efficient of friction against the inner surface of a shoe and the co-efficient of friction increases substantially when the moleskin is wet.
Blisters, abrasions, calluses, bursas and even some forms of sub-cutaneous tissue trauma are the result of applications of a combination of forceful contact and tangential tractions to the skin (forceful rubbing/forceful shearing). High shear stresses may cause damage in a single cycle. Low shear stresses may cause tissue damage when the number of cycles is great.
Tangential skin tractions relate directly to tissue shear stress and shear strain magnitudes. Shear strain is by its very nature very distortional and, when it exceeds certain levels, results in the tearing of biological tissues such as blood capillaries and interface (skin-subcutaneous) layers. High normal pressures (perpendicular to the skin surface) in the absence of significant tangential tractions are surprisingly well tolerated by skin and underlying tissue, especially when applications are of a short enough duration to avoid ischemic trauma (cell death after an extended period of blood flow blockage).
This invention is primarily aimed at reducing and preventing shear trauma from many repetitions of short duration skin loadings, but eliminating shear tractions even in low repetition, long-duration loadings is of value. Research shows that even capillary blood flow is affected strongly by whether or not shear stresses are superimposed on normal pressures. When high shear stresses are present, capillary blood flow has been shown to be occluded at normal pressures only half as great as what are required to occlude flow in the absence of shear stresses and strains.
There is some recognition among medical researchers and care-givers that shear plays a role in tissue trauma. However, how and when excessive shear stresses/strains occur and how they damage tissue are hard to visualize. Injury from a normal force (a simple, yet forceful, blow or bump causing injury by crushing tissue) is easier for people to visualize and understand. Shear stresses and how they vary over a given area (and vary with time) are very hard to measure; much harder than it is to measure normal pressure. In addition to the visualization and measurement difficulties just mentioned, there is the fact that few people have better than a vague qualitative awareness of how something called the “coefficient of friction” (C.F.) relates to blisters, abrasions, and calluses. Tangential traction force magnitudes can be no greater than the C.F. times the magnitude of normal force. Therefore, the simplest, most direct way to reduce shear induced tissue trauma is to choose materials which minimize friction against the at-risk skin surface areas. Until the present invention, there has been little practical awareness of, and attention given to, friction management.
Examination and knowledge of products on the market indicate that the opportunities for reducing callusing, blistering and abrasions by friction management has been almost entirely unappreciated by designers of shoes, orthoses, prostheses, and many other objects that come in repeated or prolonged contact with the human body.
Thin silk or synthetic fiber sheets have been used by amputees to pull over their residual limbs before pulling on a cotton or wool sock and then donning the limb prosthesis. The co-efficient of friction between the sheet and the sock is reduced under dry conditions and does protect the residual limb to some extent from friction and consequent shear-related trauma. The coefficient of friction increases substantially when the material becomes damp or wet. In most cases, the material used to line shoes and prosthesis sockets, for example, represent high friction choices. Foam products are used to line prosthetic sockets, orthoses, and shoe insoles and represent a particularly poor material from the standpoint of friction management. Damp skin and sock material literally sticks to such foams.
Synthetic gel socket liners are available, and these are generally in the range of ⅛ to 5/16 inch thick. The liner cover tends to stick to the skin and other materials in contact with it, so that it does not act as a friction reducer, but does provide cushioning and accommodates small amplitude shear motions without much resistance. The effectiveness of a gel liner is dependent on its thickness, and as it becomes thicker, its weight and bulk are deterrents.
Thus, the concept of providing a very low friction interface between the skin and surfaces that contact the skin, particularly in high load and high shear areas, has escaped the workers in the field and the need exists for reduction of trauma to the skin where the skin and tissue are supported.